Various optical devices including optical fibers, optical diffraction gratings, optical memories, and optical integrated circuits have hitherto been enthusiastically investigated and developed. Recently, polymeric materials excellent in processability, flexibility, and other properties are attracting attention as materials for these optical devices.
In producing an optical device, a technique for precisely regulating the refractive index of the device at will is indispensable. Known as such a technique for use in the case of inorganic materials, for example, is a method in which a glass doped with germanium is irradiated with a light to change the refractive index of the glass and produce an optical diffraction grating.
In the case of polymeric materials, a technique for producing an optical diffraction grating is disclosed in which a material comprising a polymer and, dispersed therein, a low-molecular compound active in photochemical reactions is irradiated with a laser light to induce a photochromic reaction (photobleaching) and thereby change the refractive index (see patent document 1). Also disclosed is a technique in which the photobleaching is used to produce a so-called graded-refractive-index material (GRIN material), wherein the refractive index changes continuously (see patent document 2).
In these techniques, a material obtained by doping with a low-molecular compound or incorporating a low-molecular compound into polymer molecules is used. There have been cases where the low-molecular compound shows enhanced light absorption and prevents the device from having sufficient transparency.
Furthermore, attempts have been made to heighten the refractive index of poly(methyl methacrylate) (PMMA), which is known as a typical polymer for optical use, by light irradiation only, without incorporating a low-molecular compound. This technique brings about a refractive-index difference of 0.051, which is sufficiently large for optical devices, through irradiation with 325-nm light. In this technique, however, methyl methacrylate as a monomer is oxidized beforehand and then polymerized, in order to impart reactivity to PMMA. There has hence been a problem that the production requires much time and the steps are complicated.
There is a report that when methyl methacrylate as a monomer is polymerized without conducting oxidation beforehand, the refractive index of the PMMA does not increase at all upon irradiation with the light (see non-patent document 1). It has further been reported that irradiation with a light having a shorter wavelength, e.g., 0.2537 μm, tends to cause cleavage of the main chain of the PMMA and lower the density thereof (see non-patent document 2). It is suggested therein that to heighten the refractive index is impossible because of Lorentz-Lorenz's formula.
(Patent Document 1)
JP-A-7-92313 (pages 2-3)
(Patent Document 2)
JP-A-9-178901 (pages 2-6)
(Non-Patent Document 1)
M. J. Bowden, E. A. Chandross, and I. P. Kaminow, Applied Optics, Vol. 13, p. 113 (1974)
(Non-Patent Document 2)
W. J. Tomlinson, I. P. Kaminow, E. A. Chandross, R. L. Fork, and W. T. Silfvast, Applied Physics Letters, Vol. 16, p. 486 (1970)